How they do this is basically to send google.com in the TLS SNI then when they get to the HTTP level send
Host: signal.org
This will only work with compatible servers. In Signal's case they use AppEngine which is behind Google's network and the servers for google.com are able to connect to their app servers when given the appropriate Host header.
Host: signal.org
This will only work with compatible servers. In Signal's case they use AppEngine which is behind Google's network and the servers for google.com are able to connect to their app servers when given the appropriate Host header.